A large part of this is that I write best if I can get my first draft down in one or two sittings, while the idea is fresh in my mind. However writing around work often means dragging things out for days, even weeks for longer stories, snatching ten minutes on the bus or an hour at lunch. I can't sustain that for anything longer than a few thousand words before I lose momentum.
The longer projects I have managed - "The Reflection of Memory", and two completed runs at NaNoWriMo - were written under very different circumstances to normal. "The Reflection of Memory" was written in an online writing group, where some friends and I got together with the goal of submitting to that quarter of Writers of the Future. There were deadlines and a supportive atmosphere. My first run at NaNoWriMo was completed on excitement and pure adrenaline, and continued with a similar online group which formed to finish novels. Unfortunately the group breaking up, and the realisation I had no idea what was behind my plot, stalled that project. My second run at NaNoWriMo ended on 30th November and 53,000 words. That one was also run on adrenaline, and also silliness as I'd promised myself a terrible fantasy trope every 1000 words.
Unfortunately the dedication required for NaNoWriMo (1667 words a day, or some seriously long weekends) just isn't sustainable in the long term, at least for me.
My work schedule gives me some long mornings, and some long afternoons (with the other end of the day being correspondingly short). I might try spending my long afternoons writing, so I can give it a couple of hours, and then use the long mornings for revising or sending submissions out. I need to try something to fit with the work hours.
I'm considering doing research in October, and then NaNoWriMo in November, although I suspect that way madness lies.